1. Field of the Invention
The present invention relates generally to a vibration damping device adapted to be interposed between members that make up a vibration transmission system in order to provide vibration damped linkage and/or vibration damped support to these members; and relates in particular to a fluid filled type vibration damping device that utilizes vibration damping action based on the flow action of a non-compressible fluid filling the interior.
2. Description of the Related Art
There are a number of known vibration damping devices designed to be interposed between members that make up a vibration transmission system in order to provide vibration damped linkage and vibration damped support to the members that make up the vibration transmission system. Such vibration damping devices will have an inner shaft fitting that attaches to one of the members that make up the vibration transmission system, and an outer tube fitting that attaches to the other member, with these fittings being elastically linked by an elastic rubber body. Additionally, one known class of vibration damping device is a vibration damping device of fluid filled design having a pressure receiving chamber and an equilibrium chamber whose interiors are filled with a non-compressible fluid, wherein the pressure receiving chamber and equilibrium chamber communicate with one another through an orifice passage. With a fluid filled type vibration damping device of this design, excellent vibration damping capabilities can be achieved based on the resonance action of fluid induced to flow through the orifice passage, and the use of such devices in applications such as automotive engine mounts, body mounts, and member mounts is currently under study.
In instances where a fluid filled type vibration damping device is employed for an application such as an automotive engine mount, the device will be subjected to input of vibration not only in the principal vibration input direction, i.e. the vehicle vertical direction, but in the vehicle lengthwise direction and vehicle lateral direction as well. Accordingly, depending on the required capabilities of the vehicle, it may be necessary for the engine mount to have different spring characteristics in the vehicle lengthwise direction versus the vehicle lateral direction. For example, it has been attempted to maintain a low spring constant in the vehicle lengthwise direction in order to reduce rumbling noise during acceleration and shock during deceleration, while establishing a fairly high spring constant in the vehicle lateral direction with a view to reducing transmission of sideways vibration which can be a problem when the vehicle is idling.
As a means for producing such different spring characteristics in two directions in the axis-perpendicular plane, there have been proposed, for example in Japanese Patent No. 4039827 and U.S. Pat. No. 6,349,927, fluid filled type vibration damping devices whose longitudinal cross sectional shape (cross sectional shape in the axis-perpendicular direction) vary in the vehicle lengthwise direction and in the vehicle lateral direction. Specifically, in Japanese Patent No. 4039827 and U.S. Pat. No. 6,349,927, there are disclosed structures in which a pair of pocket portions have been formed in zones situated in opposition to either side of the inner shaft fitting along a diametrical axis, thus imparting the elastic rubber body with reduced thickness in the portions where the pockets are formed. This has the effect of lowering the spring constant in the thinner portions of the elastic rubber body, thereby making it possible for example, by aligning the axis-perpendicular direction of opposition of the pair of pockets with the vehicle lateral direction, to concomitantly achieve the hard spring characteristics required in the vehicle lengthwise direction as well as the soft spring characteristics required in the vehicle lateral direction.
However, the fluid filled type vibration damping devices disclosed in Japanese Patent No. 4039827 and U.S. Pat. No. 6,349,927 do not provide a wholly satisfactory solution. Specifically, the fluid filled type vibration damping devices of Japanese Patent No. 4039827 and U.S. Pat. No. 6,349,927 are taught as having structures in which a pair of pocket portions formed so as to open onto the inside peripheral face of the elastic rubber body are employed as the means for localized thinning of the elastic rubber body. However, where the elastic rubber body has been thinned through formation of pocket portions in this way, it has proven difficult to ensure sufficient durability of the component while at the same time achieving the desired spring ratio in the vehicle lengthwise direction versus the vehicle lateral direction.
More specifically, in order to establish soft spring characteristics for the elastic rubber body, it is preferable to reduce the thickness of the elastic rubber body, as well as to incline the principal elastic axis (which extends in the direction of opposition of the inner shaft member and the outer tube member) by a sufficiently large degree with respect to the axis-perpendicular direction. However, where localized thinning of the elastic rubber body has been accomplished exclusively through indentation of the elastic rubber body along its inner circumference, the angle of incline of the principal elastic axis will be limited by the angle of incline of the surface of the elastic rubber body. As a result, considerable thinning of the elastic rubber body will be necessary in order to achieve the desired spring ratio in two axis-perpendicular directions, making it difficult to concomitantly achieve adequate durability. Moreover, localized thinning of the elastic rubber body accomplished exclusively through indentation of the elastic rubber body along its inner circumference will have the effect of producing extremely large changes in contours along the inside peripheral face of the elastic rubber body in the circumferential direction. As a result, stress concentrations tended to form in boundary sections between thin portions and thick portions, so reduced durability of the elastic rubber body was a problem. If on the other hand the thin portions of the elastic rubber body are made thicker with a view to ensuring sufficient durability of the elastic rubber body, it becomes difficult to achieve the desired spring ratio in the vehicle lengthwise direction versus the vehicle lateral direction.